There are no rules--it's a style preference thing. However, I prefer to use knobs on upper cabinets and handles on lowers. I find that knobs on lower cabinets tend to catch on people's pockets...

When you are "auditioning" hardware, buy a couple and try them out in your kitchen before you make the decision to buy the whole lot of them.

I can't believe I didn't think of that--my pockets catch all the time on the knobs in the lab I use at work! Thanks.